U.S. Pat. No. 6,233,818 to Finn et al. discloses a method of manufacturing of a RFID inlay. More specifically, this patent discloses a process and device for the contacting of a wire conductor in the course of the manufacture of a transponder unit arranged on a substrate and comprising a wire coil and a chip unit such as a chip module with terminal areas. By virtue of the process according to the invention described in this prior art, there is no longer any necessity, with a view to bringing the terminal areas of the chip unit into contact with the ends of the coil, to provide a separate contact substrate on which enlarged terminal areas are formed. Rather, the coil substrate, which is used as substrate for the wire coil and which, for example in the case where the transponder unit is intended to serve for the manufacture of a chip card, is formed by means of a plastic support sheet corresponding to the dimensions of the chip card, serves as a contacting or positioning aid for the relative positioning of the ends of the coil in relation to the terminal areas of the chip unit. In this case the chip unit may either be arranged in a recess in the substrate provided for this purpose or may be provided on the surface of the substrate. The first alternative affords the possibility of arranging the chip unit in the recess optionally prior to fixation of the wire conductors or of introducing the chip unit into the recess only after fixation of the wire conductors, in order subsequently to implement the actual contacting of the wire conductors on the terminal areas.
More specifically, in this prior art, firstly an antenna is applied to the substrate via a wiring device using ultrasound to attach the wire to the substrate. An antenna is thereby formed with an initial antenna region and a final antenna region, both regions traversing a window shaped substrate recess. Then, a chip module is placed in the recess whereby terminal contact areas of the module abut the initial and terminal antenna regions. Subsequently, an electrical connection is realized between the terminal contact areas and the initial and final antenna regions of the antenna by means of a thermode which under the influence of pressure and temperature creates a connection by material closure between the wire antenna and the terminal contact areas of the chip (this is also called thermo compression).
EP patent application 2 001 077 discloses a method for producing a device comprising a transponder antenna connected to contact pads and a device obtained by said process. Specifically, an antenna with terminal connections is provided in contact with a substrate. Contact pads are placed on the substrate and connected to the terminal sections of the antenna. The connection is produced by means of a soldering by introducing energy between the pads and the terminal sections. The pads are placed such as to provide a surface facing an antenna terminal connection section. The section is arranged on the substrate and the soldering energy is directly applied to the pads. A cavity is produced in the substrate close to the antenna terminal sections and a microcircuit is inserted at least partly in the cavity with contact pads positioned facing the antenna terminal sections and a soldering is carried out using thermo-compression or ultrasound. To carry out this soldering step, an anvil is used that goes through a reinforcing sheet or layer opposite the terminal section to provide a support during the soldering operation. This thus imposes the creation of a hole for the anvil which has to be carefully placed to correspond to the position of the terminal section.
The inventions disclosed in the documents cited above relate all in fact (in the practice) to the use of chip modules. Per definition, chip modules are much larger than chips and also comprise much larger connection pads. A typical chip module for contactless inlay is the mob6 from NXP, presenting a surface area of 8100×5100 μm for a thickness of 300 μm, with connection pads having a surface area of 1500×5100 μm each.
U.S. Pat. No. 5,572,410 discloses a chip being directly connected to wire antenna. In this patent, a wire is wound around a core and the two ends of the wire are soldered to metal paths deposited over the active layer of the chip. This technology, which is called “direct bonding”, minimizes the size of the resulting transponder, the number of its constituting elements as the related production costs.
The metal paths which are extensions of the usual small pads of the chips, are called megabumps (or megapads) and show a dimension adapted to the connection of the antenna wire (which shows typically a diameter of 60-80 μm). For example, a typical chip used for such applications is the Hitag μ from NXP, wherein the chip surface area is of 550×550 μm for 150 μm thickness and the megabump show a surface area of 294×164 μm (while the original pads are only 60×60 μm).
It also has to be noted that in the particular application disclosed in this document, the antenna is not embedded in a support layer, but wound around a ferrite core. The resulting transponder has a resonant frequency of about 125 kHz and the antenna show over 300 turns. This does not require a fine tuning of antenna, and the antenna spires are just wound one on the other at high speed.
However, if one wants to work at higher frequency, as for example 13.56 MHz, one will have to control the form and the relative spacing of each spire in order to tune the antenna correctly. Wire embedding is the most efficient and popular technology for manufacturing of high frequency wire antennas. But up to now, this was made exclusively by using chip modules. This introduces an important limitation as the resulting inlay cannot be thinner than the used module.
The manufacturing of thinner high frequency inlays is the main motivation to try combine direct bonding and wire embedding technologies.
Table 1 propose a list of some of the high frequency chips on the nmarket which could be used for direct bonding. These chips present much smaller dimensions (not only in thickness) in comparison to the mob6 from NXP described above.
TABLE 1examples of high frequency chips applicable for direct bondingChipBumpsBumpTotalChip dimsthicknessdimsthicknessthicknessSupplierRef.[μm][μm][μm][μm][μm]EMEM42331034 × 1054100, 200300 × 40018N/Aor 280NXPMF CLASSIC 1k650 × 675150164 × 2941816INSIDEPicopass 2k V1.21198 × 1192280310 × 7122030NXPP60D080/P60D1442166 × 300475600 × 600128VAINSIDEAT90SC28880RCFV2740 × 297075600 × 680128
The problem is to handle such small chips properly when in the same time the antenna wire is fixed on a huge sheet of plastic. Solutions used today for chip modules (which are much heavier and larger than single chips) are no more usable at such large manufacturing scale.